The laser feedback interference in gain-coupled DFB laser diodes (Distribute Feedback LD) is analyzed. The threshold gain and frequency variation in the DFB laser diode are theoretically deduced. Considering the experiment in practice, we analyze the laser feedback interference at the condition of multiple external cavities. Some parameters have influences on the feedback interference, e.g., the coupling coefficient, the linewidth enhancement factor of gain-coupled DFB laser, the reflection coefficient of external reflector. We discussed these different parameters value in numerical analysis, including the simulation at the condition of multiple external cavities. Compared with the effect of laser feedback interference on F-P and λ/4 phase-shifted DFB lasers, the gain-couple DFB lasers are more sensitive to optical feedback for a specific κL value (coupling coefficient and L cavity length). These results show that the gain-couple DFB lasers can be used as laser feedback sensors with a high accuracy.